1. Field of the Invention
The present invention relates to a method and apparatus for electrically and selectively stimulating specific nerve tissue in a living creature, namely a human being, by controlling and steering an electric field. More specifically, the present invention is directed to:
1) a spinal cord stimulator capable of electronically changing the size and/or location of the electric field by independently adjusting the current flowing through two or more anode (+) electrodes from one or more constant voltage cathode (-) electrode(s); PA1 2) circuitry for automatically changing the voltage amplitude at each anode in response to changes in the electrode impedance in order to maintain a constant anodic current, thereby preserving for the duration of the therapy the original electric field found to be most effective at implant time; PA1 3) a percutaneous stimulating lead incorporating a mechanical steering system capable of steering the lead's distal end around anatomical obstacles within the epidural space.
2. Description of the related art including information disclosed under 37 CFR .sctn..sctn. 1.97-1.99
The concept of using electrical stimulation for treating specific diseases or physical disorders is well known. Examples of electrical stimulators are: Cardiac pacemakers which restore a sick human heart to a normal rhythm, and neurological stimulators which control nerve or brain response (such as pain or epileptic seizures).
The use of a surgically implantable, electrical neurological stimulator has been well established for a number of years, especially for the control of nerve or brain response to treat intractable pain, epileptic seizures and tremors as a result of Parkinson disease. An example of a prior art device is an implantable neural stimulator powered by radio frequency (RF) or a non-rechargeable battery. Either neural stimulator incorporates the capability to designate: 1) which electrode(s) are to be used to deliver a negative stimulus pulse and which electrode(s) are to serve as the ground (return) path; 2) the pulse width of the negative stimulus pulses; 3) the voltage amplitude of the negative stimulus pulses; and, 4) the repetition rate of said stimulation pulses modulation.
A deficiency with this prior art device is that the resulting electric field (nerve recruiting area) is relatively broad and unfocused, sometimes resulting in undesirable motor responses. For the purpose of stimulating the dorsal column the stimulating electrodes are typically placed in the posterior epidural space, and the stimulus current must pass through the dura. If the electrodes have to be placed sufficiently lateral to the midline so that it is near the intercostal nerve root in order to cover the pain area, the patient will experience unwanted motor responses such as painful chest wall or abdominal wall stimulation.
Another deficiency with this prior art device is that the spinal cord movement with body position changes affect the distance between the stimulating electrodes and the spinal cord, resulting in undesirable variations in the size and/or location of the electric field, altering the paresthesia pattern. Movement of the electrodes from the optimal position can be a major problem when the patient becomes active.
With the prior art devices, where only changes of electrodes are possible and the same stimulus pulses are applied to all selected electrodes, it is often difficult to direct the stimulation field to recruit only the target nerve tissue and exclude unwanted nerve tissue.
Yet another deficiency with this prior art device is its inability to compensate for changes in electrode impedance due to; 1) growth of connective tissue around the electrodes, 2) histological changes, and, 3) changes in lead position which alters the distance between the electrodes and the dura. When a constant voltage is applied across two electrodes, the resulting electron current produces a flow of ions through the adjacent nerve tissue. A precise rate of ion flow is required for the pain relief. However, the magnitude of this electron current (and rate of ion flow) is controlled by the impedance between the electrodes. When this impedance changes, the size and/or location of the electric field also changes.
Still another deficiency with this prior art device is related to the design of the stimulation lead which makes t difficult to maneuver the lead's distal end (containing the stimulating electrodes) around anatomical obstacles normally found within the epidural space, and to force the electrodes against the dura to reduce the energy required for stimulation. During lead placement, it is not uncommon for the physician to attempt multiple passes to overcome anatomical obstacles, such as fatty tissue. Then, after the lead's distal end finally reaches the desired vertebral bodies, the physician needs to again manipulate the lead to force the electrodes against the dura to achieve paresthesia with the lowest possible stimulus energy in order to prevent unwanted motor responses.